Will Ferrell, the Saturday Night Live comedian best known for his goofy impersonation of President Bush, is leaving the show after seven years.

The SNL season finale May 18 will be his last. The California native says he is heading back to Hollywood to pursue a movie career full time. 

"If I'm on the phone with you a year from now talking about how I'm now a substitute school teacher," Ferrell told The Post yesterday, "then it didn't work out." 

Meanwhile he says he will regret leaving New York. "I'll miss just about every part of it, good or bad. I'll miss getting picked up by the police, urinating on cars in public, things like that," he said. 

Ferrell broke the news to his boss, executive producer Lorne Michaels, about two weeks ago and Michaels tried to talk him out of it. 

"Lorne's been nothing but nice about it — and yet he, of course, gave me the hard sell as to reasons why I should stay." 

Ferrell countered with an analogy he says Michaels hated: He didn't want to become that guy who's already graduated from high school but still hangs out in his van in the parking lot, picking up seniors. 

"I was pretty resolute in the way I felt," he said. "To his credit he's always let me go work on other things in the course of a season. 

"But for me it's taxing to do both. And I'm the kind of person who likes the closing of one chapter and the opening of another." 

The rest of the cast understands why he wants to leave, he says. 

Among Ferrell's first post-SNL projects are two new movies: Old School, to be released in September, in which Ferrell, Luke Wilson and Vince Vaughn play thirtysomething friends who start their own fraternity next door to a college; and Elf, with Ferrell as a human being raised at the North Pole who discovers that he isn't actually an elf. 

A third movie, Anchorman: The Legend of Ron Burgundy, in which Ferrell plays a longtime local newsman who's suddenly saddled with a female co-anchor, has yet to get the greenlight. 

At SNL, Ferrell leaves behind a big hole. 

Among his best-known SNL characters was President Bush, former Attorney General Janet Reno and a dead-on impression of Alex Trebek, the brainy host of Jeopardy!

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